A silicon high-rate etching process where silicon is etched using inductive plasma excitation is discussed in German Published Patent Application No. 42 41 045. To achieve as high as possible etching rates it may be required to provide an as high as possible concentration of fluorine radicals in the plasma produced. This may occur via irradiation of an appropriately high high-frequency power, having power values between 3 and 6 kW, into the inductive plasma source under simultaneously increased process pressure of 30 to 100 μbar, or up to 250 μbar if the required uniformity of the plasma produced permits this. However, an undesirable ion density may appear in such processes along with a desired increase in the fluorine radical density and these ions may interfere with the actual etching process and may be detrimental to a sufficiently high mask selectivity. These ions heat up the wafer to be etched, resulting in profile deviations. By using a suitable device in the plasma etching system it may be subsequently ensured that the density of the ions produced is reduced to acceptable low values and it may be homogenized. In this connection it has been proposed that a recombination of ions and electrons be made possible by using diffusion paths or at aperture constructions, according to German Published Patent Application No. 197 34 278 for example. The portion of the high frequency power injected into the plasma which was used to generate an undesirably high ion density may be lost here in the form of heat or radiation.
Although other prior devices for the reduction of the ion density may be successful, in practice there may still appear, in particular in the margin area of the substrate to be etched, profile deviations, profile asymmetries, etching rate excesses, or regular or irregular profile undercuts, which may be referred to as “beaking” effects and which may not be tolerable in many applications.
In other prior devices for the reduction of the ion density it may be a problem in particular that an ion density in excess by one to two orders of magnitude may be required to be applied completely homogeneously to the desired area over the entire surface to be etched, having, for example, a diameter of 150 mm or 200 mm, without thereby causing distortions of the plasma potential on the surface of the substrate to be etched. Due to such distortions of the plasma potential, excessive local ion densities and/or excessive local ion energies may result there in the above-mentioned detrimental effects.
German Published Patent Application No. 199 00 179 further refers to a plasma etching system for high-rate etching of silicon using inductive plasma excitation in which the plasma source, having a balanced supply of the inductive excitation inductor, is provided. Such a plasma etching system, having a symmetrically configured adapting network for adapting the impedance of the injected high frequency power to the impedance of the plasma produced and the simultaneously balanced supply of the inductive excitation inductor may result in a minimal injection of undesired displacement currents into the plasma and thus in minimal distortions of the plasma. Such a symmetrical inductor supply may be desirable at high power values of more than 3 kW.